Frame 9E gas Turbine Axial probe

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Thread Starter

Chandra

We are having a GE frame 9E gas turbine, which is under major outage.

The shaft is removed and is back after the maintenance. In bearing No:1, we are using axial probes which connected to Bently 3500 system. The voltage setting mentioned in device summary is -10.5 volts. How to adjust the axial probe? Do we need to move shaft with help of mechanical, while adjusting the probe?

The mentioned -10.5 volts is referred to which position?

Kindly advice.
 
I've only participated in doing this once, and it was such a contested process, with everyone who was involved in the procedure had a different idea about how it should be done.

One thing is for sure: You will need to use some mechanical means (usually a hydraulic jack) to move the shaft back and forth between the active and inactive thrust faces. It's typically called "bumping the rotor", or a "thrust check."

The GE Clearance Diagram should list the allowable axial movement between thrust faces.

The big controversy when I was involved with the procedure was: Was the "zero" position of the shaft against the active thrust face? Or was it in the middle of the allowable thrust movement? Or was the zero position with the shaft against the inactive thrust face? And everyone had an opinion.

I believe whatever "zero" position you choose, the operators and their supervisors and the instrumentation and control technicians all need to know what that choice is. And, everyone needs to understand that thrust measurement is all about trending, meaning that it's about the change in reading with respect to time. Is the thrust measurement increasing or decreasing? And how quickly is it increasing or decreasing?

As for the -10.5 volts, I think that's the adjustment made on the 3500 monitor for the total movement of the shaft between thrust faces.

My belief was that the zero position was with the shaft against the active thrust face, and if the active thrust face or the shaft wore down then the reading would increase from zero in a positive direction. An increase in the negative direction would mean the shaft was moving away from the active thrust face and toward the inactive thrust face, which for a Frame 9E should not happen when the turbine is running. (F-class units, as I recall, actually move toward the inactive thrust face during start-up and acceleration, and then when near rate speed the shaft moved back to the active thrust face.)

But, I was over-ruled by the Operations Supervisor who believed the zero position was with the shaft in the middle of the thrust movement. And I do know that to this day (and that was about six years ago) they are still questioning why there is such a large thrust indication when the unit is running. I don't think anyone remembers what the initial thrust reading was when the unit was first started after the setting of the thrust probe, so no one really knows if the shaft has moved from that position. But EVERYONE questions why the thrust reading is so "large" when the unit is running, and if there is some problem with the thrust bearing.

So, make sure everyone is in agreement with the zero position, and that the initial position when the unit is first started after the outage is recorded for posterity so that changes can be referenced to the original position.

Lastly, if I recall correctly, I believe there is some instructions/information in the Bently-Nevada manual for the axial position (thrust) probe about setting the range (span). But, I don't think is specifies what the zero position is or should be. I believe the zero position is dependent on the application of the axial position probe (which could be for thrust movement, or for casing movement, or for relative movement, etc.).

Hope this helps! Perhaps others who have experience with setting and adjusting axial position probes being used to measure thrust movement on GE-design heavy duty gas turbines can speak from their experience.
 
My experience on the steam side is about the same. Everyone has an opinion on where it should be set.

The other information I like to consider is how they think the alarm and trip should be set. If the active/inactive values are equal in magnitude, then I would think "zero" is in the middle. You definitely do not want an alarm inside of your "bump" range. The legacy mechanical hydraulic design devices were "zero in center"

For the few proximity probe type thrust monitoring devices I have assisted with the alarm/trip were equal magnitude. the rotor was bumped and the thrust clearance noted. then bumping to one plate, the probe gap voltage was set to the "zero" voltage PLUS or MINUS the calculated voltage for half of the bump travel.
then bumped to the other plate and verified zero voltage MINUS or PLUS.

For a unit with 0.015" bump, the running value will be very close to 0.007" (active side polarity verified during calibration). if it is more than that, then the mounting configuration needs to be evaluated or the thrust retaining
 
Chandra,

I think I like JFBjr's approach the best, after giving it much consideration.

If it's not too late, you should use his method, and, again--be CERTAIN to inform everyone (operators, supervisors, technicians) exactly how it was done, and provide a written drawing of the results of the process which can be used by everyone in the future. (Writing a procedure for doing the exact same "calibration" in the future would also be extremely beneficial to everyone on site.)

> If the active/inactive values are equal in magnitude, then I would think "zero" is in the middle. You definitely do not
> want an alarm inside of your "bump" range. The legacy mechanical hydraulic design devices were "zero in center"

> For the few proximity probe type thrust monitoring devices I have assisted with the alarm/trip were equal magnitude.
> the rotor was bumped and the thrust clearance noted. then bumping to one plate, the probe gap voltage was set to
> the "zero" voltage PLUS or MINUS the calculated voltage for half of the bump travel. then bumped to the other plate and
> verified zero voltage MINUS or PLUS.

Regardless, it is necessary to mechanically move the rotor back and forth to obtain the actual thrust distance (between thrust faces) and to calculate the values as suggested by JFBjr.

Please write back to let us know how you proceed, and what the results of the verification of your procedure were.
 
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